As a consequence of the complexity of aircraft instrumentation systems, pilots are burdened with a significant amount of monitoring activities, even during normal operations. To this end, the pilot frequently needs to lower his head to obtain flight information from the cockpit instruments. Such information will typically include many discrete bits of data which need to be checked repeatedly. These include, for example, torque, altitude, heading, attitude and a vast array of other similar pieces of information. One of the more burdensome tasks which a pilot is oftentimes required to accomplish is to fly the aircraft along a preselected flight path having various waypoints or flight plan benchmarks mapped out along the preselected route. This task requires consulting various cockpit instruments and maps and comparing the present aircraft position with what is shown on the map and with what may be seen outside of the cockpit windscreen. Flight directors have been developed to ease this burden. However, for flying in a nap-of-the-earth mode, the pilot cannot afford to divert his attention to any in-cockpit instrument, lest he be surprised by an unexpected obstacle or threat in his path.
Electro-optical equipment has been developed to at least in part reduce the frequency of the need to look down by superimposing symbolic flight information similar to what appears on the instrument panel into the pilot's field of vision. The images are typically presented to the pilot's eye by means of collimated light rays so that the symbology appears to be at optical infinity and the pilot need not accommodate to view the individual symbols. Thus, the pilot views the real world before him focused at infinity while, at the same time, he sees the symbols, also at infinity. The superimposition of the two images, i.e., the real world and the electronically generated symbology, enable the pilot simultaneously to assimilate pictorial information from the outside world and informational symbology without having to look inside the cockpit nearly as frequently. However, complex tasks such as staying on a flight plan may still require map consultation and consume large amounts of time.
The first generation headup displays consisted of a cathode ray tube (CRT) with appropriate optical elements mounted in the aircraft's instrument panel. The CRT generated an image which was provided onto a combining mirror for viewing by the pilot. Selection of collimating optical elements between the CRT and the combining mirror caused the projected images to appear as if at infinity. In spite of the obvious advantages, the first generation headup displays had three major problems. First, the displayed information was stationary with respect to an axis, usually aligned along the longitudinal axis of the aircraft. Second, large amounts of scarce instrument panel space was required for the large and bulky CRT. Third, the images were presented within a limited field of view.
A second generation headup display was developed for helmet mounting of the CRT and optics See, e.g., U.S. Pat. No. 3,923,370, granted to Mostrom, at column 4, line 58 to column 5, line 42. Unfortunately, such CRTs were necessarily small in size, were necessarily operated at safe voltages that were less than optimal for brightness purposes and generally produced dim images with poor resolution. Despite the small size, heat and weight problems associated with the CRT contributed to pilot fatigue. The optics in the various second generation headup displays varied widely in attempts to find the best way to generate and project symbology images in the form of collimated rays into the eyes of the pilot. See again U.S. Pat. No. 3,923,370, column 5, lines 1-42. See also, for example, U.S. Pat. No. Re 28,847, reissued to Vizenor, at column 3, lines 58-65, column 4, line 52 through column 5, line 3.
A third generation of helmet mounted headup displays was developed to provide a more efficient design. The CRT was removed from the helmet and placed in a noncritical portion of the cockpit with an optical fiber bundle coupling the CRT with the pilot's visual faculties. See U.S. Pat. 4,439,755 to LaRussa, column 3, lines 1-5. See also column 1, lines 14-15 where "enhanced or computer-processed data base images of the terrain" are mentioned and column 2, lines 20-31 where helmet monitoring is suggested for target acquisition purposes. Still another third generation approach is described in copending application U.S. Ser. No. 079,553 entitled "Method and Apparatus for Mounting a Cathode Ray Tube for a Heads-Up Display System" filed by Smith on July 30, 1987 and owned by the assignee hereof.
Thus, a headup display system will typically include an image source such as a CRT which provides images of various symbols for the representation of information generated by an electronic computer. From the image source, the light rays travel through an optical system of one sort or another onto a combining element situated in the pilot's field of vision either on a helmet or interposed between the pilot's head and the front of the wind screen, which element transmits real world images and reflects symbology images by means of collimated light into his eyes.
Depending on the flight mode, the pilot can typically select various operating modes of the display system so that only those graphics, symbols and alphanumerics needed in each mode are displayed, such as, e.g., landing, weapon release and navigation. The symbology used has not been standardized in the industry because improved displays are continually being developed.
A general display mode might include indications of airspeed, altitude, angle of attack, vertical speed, heading, cross-track distance, artificial horizon, a flight director (indicating, together with the artificial horizon and the instantaneous flight attitude, the course corrections needed to stay on a preselected flight plan), pitch (by means of, e.g., a separate pitch ladder) and a separate roll angle indication. See, for example, U.S. Pat. 4,305,057 to Rolston in which a headup display provides a forward looking view along the longitudinal axis of the aircraft to the pilot as if the aircraft were gimballed inside and at the center of a transparent sphere which has heading and pitch angles marked on its surface.
For a military aircraft, an air-to-ground display mode for weapons delivery correction might be selected in preparation for the more accurate destruction of an impending target. A waypoint, within view on the way to the target, might be identified on a map and would have its coordinates manually entered by the pilot into the navigation system. The pilot would then manually align a marker on a headup display into coincidence with the actual waypoint and a more precise fix may then be made on the position of the aircraft with respect to the waypoint and ultimately, the target. Various air-to-air modes are also known in the art of military headup displays including computed lead angle mode (to directly enable positioning the aircraft in the best possible firing position) and trajectory mode (taking into account the laws of ballistics as well as the aircraft's speed to compute the shell trace which would result if the guns were fired [the pilot can also obtain a lead angle by maneuvering the aircraft such that the shell line goes through the target]).
Even for non-headup display systems the pilot's mental burden can be greatly alleviated using innovative display symbology. A landing display, as shown in U.S. Pat. No. 4,368,517 granted to Lovering for an "Aircraft Landing Display System," is representative. Lovering discloses a non-headup display fixed in the cockpit for imaging a runway in the correct perspective to enable the pilot to ascertain his position with respect thereto, particularly under low visibility conditions, and further, to ascertain the consequences of various corrective maneuvers. The display includes a horizon reference line symbol fixed in a horizontal position with respect to the frame of the display. Below this horizon reference line a runway symbol is presented in size and perspective as if representing the actual runway viewed from the aircraft. These two symbols are combined with an aircraft symbol below the runway symbol to provide a real world display from which the pilot can ascertain the attitude of his aircraft and the progress of his approach. Lovering describes his display as providing the pilot a view of the runway, horizon and aircraft as if viewed from a position in space detached from the aircraft.
Lovering's display tries to recreate external visual cues inside the cockpit. "Contact" flight means that the pilot is flying the aircraft utilizing such external visual cues which he can see outside the canopy, e.g., the horizon, sky, clouds and objects on the earth. Contact flight is distinct from "instrument" flight which is generally used under adverse visibility conditions. Although meaning different things to different persons, "contact-analog" can be used to mean, in a very general way, the provision of visually analogous information which is simulative of "contact" flight. Defined thusly, Lovering's airport runway display is of the contact-analog type.
There exist two basic categories of contact-analog displays: "outside-in" and "inside-out." For example, an "outside-in" type of contact-analog vertical gyro indicator is made up of a circular dial on the instrument panel having the horizon presented immovably in a horizontal position etched across the dial while a smaller line with a circle in the middle, symbolically representing the aircraft in section, tilts on a moveable axis in the center of the dial with respect to the artificial horizon in order to indicate the roll attitude of the aircraft The symbol rises or lowers with respect to the horizon to indicate pitch. Lovering's airport runway display may be classified as of this type also. Outside-in displays give the pilot an acceptable "feel" for the relation between his control actions and their effects. The pilot is enabled to think of himself, for example, as observing his aircraft in a detached way from the perspective of a chair situated at a point in space outside, above and to the rear of the canopy. The chair has a fixed attitude with respect to the earth. Thus, the "outside" world is brought "in" to the cockpit.
For an example of the other type, an "inside-out" type of contact-analog vertical gyro indicator always shows the aircraft symbol immovably and horizontally with respect to the instrument panel while the horizon line tilts on the dial to indicate aircraft roll and moves up or down to indicate pitch. Rolston's aircraft attitude reference display may be classified as of this general type. Inside-out displays are widely used because they convey, as opposed to outside-in displays, a better "feel" for the orientation of the aircraft in relation to the horizon. For the inside-out vertical gyro indicator, the pilot sees the artificial horizon tilting with respect to the fixed aircraft symbol in much the same way as he sees the actual horizon "tilting" outside the canopy with respect to the aircraft fuselage Rolston takes this concept one step further by having the pilot thinking of himself as if in an aircraft gimballed on a three axis platform at the center of a transparent "sphere" having heading and pitch angle symbology fixedly etched thereon. The "sphere" translates along with the aircraft but, unlike the aircraft, keeps its attitude stable with respect to the surface of the earth.
A recently developed system for aircraft pilots, more particularly for helicopter (Apache) pilots, is described in U.S. Army Publication TM-55-1520-238-10, pp. 4-19 through 4-24. Therein is described a helmet mounted display system suitable for presenting certain flight information to a helicopter pilot including symbols indicative of aircraft operating and aircraft flight parameters such as engine torque, aircraft airspeed, rate of climb and a horizon indicator. These indications, along with an attitude indicator are presented before the pilot's eyes so he doesn't have to look down. The indicator is of the inside-out type.
As mentioned, both the outside-in and the inside-out approaches have their advantages The disadvantages for each approach arise because of the mental vantage point peculiar to each, not quite real, which the pilot should assume in order to properly interpret the display using a given cognitive faculty. The mental assumption of the artificial vantage point results in the emphasis of perceptions obtained using the given cognitive faculty. The assumed vantage point disjoints the emphasized perceptions from related perceptions which are normally felt along with the emphasized perception. This is due to the related perceptions being not necessarily best interpreted from the assumed vantage point. There may even be a conflict between perceptions obtained from normally harmonious faculties.
For example, to properly interpret an inside-out vertical gyro indicator the pilot has to align his visual axis with the longitudinal axis of the sectional aircraft symbol and mentally identify the attitude of his body with that of the fuselage. Similarly, in order to properly interpret the markings on Rolston's "sphere" the pilot has to think of his body, with head erect and facing forward, as being rigidly "at one" with the aircraft. This is due to the association and hence the orientation of the display information with the longitudinal axis of the aircraft. For the Apache head-mounted display, described above, the pilot should also think this way but may become somewhat disoriented if he uses the attitude indicator after turning his head away from forward. In all these cases, the fact that it is the pilot and not the aircraft that is perceiving the symbology is ignored and the simulative efficacy of this particular inside-out "contact-analog" technique is thereby adversely affected to a significant degree.
Similarly, Lovering's outside-in display requires the pilot to assume the role of a detached observer who, although supposedly viewing a perfectly stable and horizontal horizon as shown for example in position 8 of FIG. 2b of U.S. Pat. 4,368,517, is nevertheless physically experiencing the aircraft roll effect depicted by the aircraft symbol in the Figure. The angular acceleration forces the pilot actually experiences conflict with those which would be experienced by the detached observer whose vantage point he is at the same time trying to mentally assume.
In view of the nature of our invention, as disclosed in detail below, it will become apparent that the problem with the prior art contact-analog concepts described previously, and this goes to the heart of our invention, is that they all decouple the pilot in one way or another from that which is displayed, i.e., from either the aircraft (outside-in) or the real world (inside-out). In other words, the present state-of-the-art contact-analog displays use the aircraft or the earth as the referent and the pilot has to mentally assume that the referent really is, from his point of view, a stable reference which is not always true. In retrospect, based on a knowledge of the teachings disclosed herein, it would therefore have been more advantageous had the prior art presented such contact-analog information in a manner which avoided decoupling the pilot from what was displayed.